I'm adding some extra phone/network connections in the spare bedrooms of my house. What is the difference between Cat5e CMP cable (Plenum, blue color) and CMR (Riser, brown color)? Is it just fire rating?

The plenum cable is fire rated differently as it emits less smoke / toxic fumes when it burns. It's quite expensive compared to CMG or CMR cable, either of which are suitable for residential in-wall application. I would not waste my money on the CMP, but I would use it if someone gave me a spool.